Air drying solvent based alkyds are generally prepared by transesterification and condensation reactions of three types of monomers: polyalcohols, polybasic acids and unsaturated fatty acids or the corresponding oils. In the chemical curing process of alkyd binders, the presence of unsaturated fatty acid chains is essential. Polymeric network formation takes place by the crosslinking of the fatty acids moieties of the resin. The number of unsaturated bonds in the fatty acids determines whether the oils are classified as non-drying, semi-drying or drying oils. The architecture of the fatty acids, such as whether or not the double bonds are conjugated and the configuration of the double bonds, has also an effect on the drying speed.
The chemical curing of air-drying systems is a rather complex process composed of oxidation reactions, radical formation and crosslinking leading to a polymeric network.
Since the drying process proceeds slowly, metal based driers are added to catalyze the reaction. Driers are typically metal soaps of either transition metals or alkaline-earth metals. Driers are classified by function, specifically: surface driers, through driers and auxiliary driers. Surface driers are characterized by having at least two accessible valence states and catalyze the autoxidative curing process. Through driers promote curing beneath the surface of the coating film and auxiliary driers interact with the surface driers.
A liquid coating composition containing unsaturated fatty acids and metal driers, when exposed to air, will rapidly convert into a drying polymeric matrix. The formation of surface skin on air-drying coatings results from the same drier catalyzed oxidative polymerization processes. Even in a closed container, in which the ullage between the product surface and the closed lid is sufficiently large, a tough rubber-like skin on the surface of the liquid coating material will develop. To prevent untimely oxidative drying leading to skinning of the alkyd paint and to improve the stability during storage, anti-skinning agents are included in the formulation.
Skinning in the paint container during storage results in loss of quality and quantity of the coating product. Removal of the skin is both awkward and time consuming and results in a waste of a substantial amount of paint. Moreover, the concentration of driers found in the skin is disproportionate and removal will lead to a prolongation of drying time of the remaining coating material.
Anti-skinning agents prevent early oxidative drying by binding oxygen, acting as radical scavengers or by complexing to and thereby inactivating the metal drier. The different types of anti-skinning agents described in the literature include substituted phenols, hydroquinones, aliphatic and aromatic amines, tin compounds, azones, α-hydroxyketones, hydroxylamines, β-dicarbonyl compounds, natural antioxidants such as tocopherol and isoascorbates, solvents as dipentene and oximes.
The choice of the anti-skinning agent is always a compromise between preventing skin formation and retaining an adequate drying potential of the coating after application but also odour, toxicity profile and the effect on the coloristic film properties should be taken into account.
For decades, the class of oximes has been the most widely applied group of anti-skinning agents in the coating industry as a result of well-balanced properties. Particularly methyl ethyl ketoxime (MEKO) is widely used in coating formulations because of having no adverse effect on the drying time by virtue of the high volatility, long-lasting anti-skinning control in a closed container, no discoloration of the paint film, high compatibility with a wide range of coating materials and a mild odour. The mechanism associated with the anti-skinning performance of oximes is still not accepted unanimously. One of the widely encountered hypotheses concerns the association of the oxime to the free coordination sites of the metal carboxylate thereby suppressing the activity of the catalyst. During storage in a closed container the weak metal-oxime complex will stay intact. However, after opening of the can or paint application, the complex will dissociate and the relative volatile oxime will be released into the surrounding atmosphere. The equilibrium of the metal-oxime complex is shifted and the inactive oxime-drier complex is disintegrated thereby restoring the catalytic functionality of the primary drier. Alternative mechanisms suggested for the mode of actions of oximes as anti-skinning agents include the scavenging of radicals formed in the autoxidation reaction or binding free oxygen. A recent study suggests that the free radicals formed in the autoxidation reaction may readily add to the C═N double bond of the oxime, producing stable radical addition products that could inhibit further free radical chain reactions. After application and exposition of the paint to air, the addition products decompose by reaction with oxygen releasing the free radicals and the oxime evaporates.
In addition, volatile oximes vaporized in the headspace of a paint container contribute to the anti-skinning performance by providing a blanketing vapor at the paint-air interface.
Because of the volatility of MEKO, a high concentration of vapor is formed covering the liquid paint in a closed container, preventing the ingress of oxygen and the autoxidation reaction.
Discussions are ongoing to reduce the workplace exposure limit (OEL) of methyl ethyl ketoxime substantially which would impose the reduction of the concentration in a coating formulation below a technical successful level.
In the patent literature, examples of oxime-free anti-skinning agents have been described. These typically result in a poorer drying performance and reduced film hardness values.
Thus, there still exists a need for an oxime-free or strongly reduced oxime containing anti-skinning agents which in term of properties profile as long-lasting resistance to undesirable skinning, effect on drying performance, odour and tendency of discoloration of the final film is comparable to methyl ethyl ketoxime.